[metacity] bump GTK+ required version to 3.20.0

[Alberts Muktupāvels <muktupavels src gnome org>]

commit 6b81655ed087951f2857b2da252a88557f5e2277
Author: Alberts Muktupāvels <alberts muktupavels gmail com>
Date:   Thu Apr 20 14:47:37 2017 +0300

    bump GTK+ required version to 3.20.0

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 379598d..1a7e54f 100644
--- a/configure.ac
+++ b/configure.ac
@@ -89,7 +89,7 @@ dnl Check for required packages
 dnl **************************************************************************
 
 GLIB_REQUIRED_VERSION=2.44.0
-GTK_REQUIRED_VERSION=3.19.8
+GTK_REQUIRED_VERSION=3.20.0
 PANGO_REQUIRED_VERSION=1.2.0
 XCOMPOSITE_REQUIRED_VERSION=0.3